The geological setting and formation of the Rosebery volcanic hosted massive sulphide orebody Tasmania

The Rosebery pyritic zinc-lead-copper-silver-gold orebody is the major massive sulphide deposit associated with the Cambrian Mount Read Volcanics of western Tasmania. The Mount Read Volcanic belt in the Rosebery area is composed principally of rhyolite and is in fault contact with the Rosebery Group to the west. The latter group, now preserved as a number of fault slices, consists predominantly of marine sedimentary rocks, deposited below wave base, and derived partly from the Mount Read Volcanics and partly from a Precambrian metamorphic terrain (the Tyennan Geanticline) further east. The Rosebery Group sediments were probably deposited contemporaneously with part of the Mount Read Volcanics and are supposed to be correlative with the Middle to Late Cambrian Dundas Group. Ore formation occurred in a marine environment, following deposition of a thick, mainly welded, ignimbrite and attendant subsidence. The orebody consists of a folded, discontinuous massive sul phide horizon and in the southern part of the mine there is a normal across-layer metal zonation with Fe-Cu-rich ore overlain by Zn-Pb-Ag-rich ore. A complementary lateral metal zonation is present with a lower and central Fe-Cu-rich zone surrounded by a Zn-Pb-Ag-rich zone. Barite-sulphide ore occurs as lenses higher in the sequence separated from the sulphide orebody by unmineralized cleaved siltstone. Overlying rocks include siltstone with a minor component of volcaniclastic sandstone and turbidites derived from the Tyennan region. Succeeding pyroclastic units are unwelded and, in contrast to rocks in the footwall of the orebody, contain quartz phenocrysts and rip-up mudstone clasts. Later feldspar-phyric rhyolite marks a change to less explosive volcanism and probably a return to subaerial conditions. A broad alteration zone of quartz + sericite ± pyrite underlies the Rosebery ore deposit. Flanking alteration assemblages include partial carbonate - and sericite - replacement of plagioclase phenocrysts in both hanging wall and footwall volcanic rocks. Towards the massive sulphide ore chlorite and pyrite increase in abundance in the footwall rocks with chlorite and chalcopyrite being more prominent below eu-rich sections of the then lenticular orebody. Chemically footwall alteration resulted in a general increase in Fe, Mg, Rh, K, Mn and H 0 and a marked decrease in Na, 5r,and generally, Ca. The Co2 content of pyrite is highest under eu-rich sections of the orebody. 034 S values of sulphide minerals show good correlation with the lateral metal zonation, but are largely independent of the across-layer zonation. 0 34 S values range from 8.0 to 13.4%0 in the Fe-eu-rich lateral zones, to 15.1 to 17.2%0 in the major Pb-Zn-rich zone at the southern end of the orebody. This pattern is paralleled by a decrease in the Fe/Fe + Mg + Mn ratios of chlorite and a decrease in the FeS content of sphalerite. The 035 S values of sulphides in the barite orebody are even higher at 14.5 to 19.81<0 and the Fe5 content of sphalerite is lower. Barite in the barite orebody has a range of values of 34.6 to 41.21<0. The 0 34 S values of barite-sulphide pairs indicate a temperature range of 2550 to 2980 e for the barite orebody, but deposition of this ore is believed to have occurred at about 2500 e. The presence of premetamorphic arsenopyrite in the sulphide orebody suggests deposition of this ore was from solutions in which H 5 was the 2 dominant sulphur species. This sulphur was probably derived from a mixture of totally reduced seawater sulphate and sulphur derived from leaching of the underlying rocks. The 034 S value of the barite orebody may represent partial reduction of seawater sulphate. The orebody and enclosing rocks were deformed and metamorphosed to the lower green schist facies in the Devonian, with textural evidence suggesting that the peak of metamorphism post-dated cleavage formation in the rocks. The formation of tare biotite and spessartine, corrosion of arsenopyrite by tetrahedrite-tennantite indicate that the peak metamorphic temperature may have been around 350o C. Chlorite compositions indicate equilibration at temperatures of about 250oC, probably during retrograde metamorphism. The use of 03'S values of sulphides as stratigraphic markers indicates that the sulphide orebody is diachronous. The lateral metal and sulphur isotope zonation may be related to discharge of ore solutions as a plume showing reversing buoyancy during formation of the Pb-Zn-rich ore, or a shift in the locus of solution discharge with time! or a 'comb1nation of these factors. The ore fluid is believed to have been generated by thermally-driven convective circulation of seawater, the energy source probably being a Cambrian granitic pluton.